This invention relates to a hoist, and more particularly to a hoist in which a hand-driven or motor-driven load sheave moves a chain engageable therewith to lift or lower a load.
Generally, this kind of hoist has a mechanical brake provided at a drive shaft which transmits power to the load sheave from a motor or a hand wheel having a hand chain, and an overload protection device provided on a power transmitting route.
The mechanical brake comprises a disc hub fixed to the drive shaft, an anti-reverse-rotation plate restrained from rotating in the direction of lowering the chain, a driving member screwed with the drive shaft, and friction plates interposed between the disc hub and the anti-reverse-rotation plate and between the anti-reverse-rotation plate and the driving member. The overload protection device is separate from the mechanical brake and is provided on the power transmitting route.
The hoist, for safety, is rated at its maximum load, i.e., the maximum weight of a load which can be handled, so that a worker is always instructed to use the hoist within its normal rating.
However, a worker is not always familiar with the precise weight of a load handled by him. He occasionally handles a load over the normal rating of the hoist, causing the load sheave and chain and a reduction gear train to be overloaded and a very dangerous break down of the hoist. To prevent this, the hoist is always equipped with an overload protection device.
Conventionally, the overload protection device is separate from the components of the hoist and comprises a torque limiter, e.g., a slip gear, so that when the hoist lifts a load over the normal rating, the slip gear slips to check the load lifting. The torque limiter which is separately provided, makes the hoist complex in construction and larger in weight. Furthermore, it is possible that the load, once suspended, will fall due to a slip of the slip gear.
In light of the problems, this invention has been designed. An object of the invention is to provide a hoist which is simple in construction, has only a slight increase in weight, and has an overload protection means which is of high efficiency and which is free from the possibility of a suspended load falling even when an overload condition actuates the protection means.
This invention has been designed to utilize the mechanical brake at the hoist for overload protection. The present invention is characterized in that the drive shaft of the hoist is provided with a spline and a screw thread, the spline supporting a disc hub axially movably, the screw thread screwing with a driving member, the disc hub being disposed opposite to the driving member; an anti-reverse-rotation plate unidirectionally rotatable is interposed between the opposite surfaces of disc hub and driving member through friction plates, and is supported to the disc hub; a pusher biasing the disc hub toward the driving member is provided at one axial side of the disc hub; and between the disc hub and the hoist body is provided a locking mechanism which is actuated by axial movement of the disc hub to thereby stop the rotation thereof.
Forward screwing of the driving member at the mechanical brake is used to move the disc hub axially to thereby actuate the locking mechanism between the disc hub and the hoist body. Hence, the rotation of disc hub, and in turn, of the load sheave, is locked to hold the suspended load in the predetermined position which prevents it from falling when the hoist is overloaded in excess of the normal rating. Consequently, a simple construction addition to the hoist makes it possible to provide overload protection having a high degree of efficiency and safety.
This invention is applicable to both a hand-operated hoist and a motor-driven hoist housing therein an electric motor. In the hand-operated hoist, a hand wheel engaging with a hand chain functions as the driving member, and in the motor-driven hoist, a second gear of a reduction gear train which transmits power from the motor to the load sheave functions as the driving member.
In the hand-operated hoist the locking mechanism is provided between the disc hub and a base plate constituting the hoist body. In the motor-driven hoist the locking mechanism is provided between the disc hub and a stationary member fixed to the base plate.
These and other objects and novel features of the invention will be more apparent from the following description of an embodiment thereof provided in conjunction with the accompanying drawings.